In many fields, the skilled person is confronted with the need to separate one or more organic compounds from a mixture of organic compounds sharing similar structures and properties but differing in the number and/or position of the double bonds. As the individual compounds with different degree of unsaturation in such mixtures may differ only slightly in their physical properties, exhibiting similar molecular weight, boiling point and solubility, separation procedures based on these properties are generally ineffective to obtain high purities or highly enriched fractions of one or more of the different organic compounds. Some examples include the separation of unsaturated aliphatic hydrocarbons (which find use as the reactive monomers for preparing synthetic polymers) from hydrocarbon mixtures in which they are found and the separation of mixtures of saturated and unsaturated carboxylic acids, such as mixtures of fatty acids or esters thereof, into the individual components or into mixtures enriched in one or more of the individual components.
For instance, fatty acids or fatty acid esters present in mixtures are generally difficult to separate, because they possess similar molecular weights and the same set of functional groups. In this context, fatty acids typically possess a carboxylic acid functional group and 0, 1, 2, or more carbon-carbon double bonds. The lack of an economically viable separation or fractionation method limits the application of fatty acids, such as e.g. as a starting material for industrial applications. Indeed, fatty acids present in oil or fat occur generally as a mixture of saturated and unsaturated fatty acids which reflect the fatty acid composition of the oil or fat, which is dependent on the source of the oil/fat. It is noted that there are specific uses for fatty acid materials having, respectively, higher or lower degrees of unsaturation. Depending on the application, a fatty acid (ester) product becomes more valuable as the proportion of saturated or unsaturated components is increased in the product. Thus, it is often desirable to be able to ensure a selective recovery of fatty acids.
Membrane-based separation methods are inexpensive, easy to implement, and readily scaled up to separate many tons of material. Membrane separation is typically one of the simplest and least energy intensive methods of purification. However, membranes are usually not suitable for the separation of mixtures comprising saturated and unsaturated compounds having similar properties and structures, such as size and polarity. For instance, separation by nanoporous membranes requires organic molecules with large differences in molecular weight, wherein the larger molecule is mostly retained while the smaller molecule passes through the membrane.
There is a need in the art to develop inexpensive and practical separation methods and tools which allow to separate or fractionate mixtures of organic compounds having very similar structural and physical properties but differing in the number and/or position of unsaturated (double) bonds, into the individual compounds or into mixtures enriched in one or more of said organic compounds.